According to a generally used method of starting pumping operation, a static starter equipped with a frequency converter including a semiconductor element is used for starting of generator-motor at a large-capacity pumped storage power plant. This starter is used to accelerate the generator-motor to a rated rotating speed from the standstill state to synchronize a generator-motor terminal voltage with a system voltage, and then the generator-motor is connected in parallel with the electric power system by a parallel-in breaker. A thyristor starter is known as such a static starter. The output of the starter of this type is as high as 20 MW, and there are demands to reduce its costs and installation space.
On another front, in an environment marked by the recent countermeasures against global warming, there are increasing cases where a doubly-fed generator-motor (hereinafter referred to as a “variable-speed generator-motor”) is applied to a variable-speed pumped storage power plant. When a large-capacity variable-speed generator-motor is applied to a variable-speed pumped storage power plant, a frequency converter (hereinafter referred to as a “secondary exciter”) having an output of more than 20 MW may be used for an exciter connected to the rotor winding (secondary winding) of the variable-speed generator-motor. Some variable-speed pumped storage power plants use the following method (hereinafter referred to as a “self-starting method [secondary side]”): The “secondary exciter” is used as a starter without an additionally installed static starter. Variable-frequency driving electricity is put into a variable-speed generator-motor from a secondary winding of the generator motor to accelerate the generator motor to the vicinity of a synchronous speed from a standstill state.
In the “self-starting method [secondary side]”, the target speed of synchronizing with and connecting in parallel with the electric power system is greater than or equal to the minimum speed of what is known as a variable-speed operation range.
However, the selected variable-speed operation range of the recent variable-speed pumped storage power plant has decreased to about ±4% from a conventional range of about ±5 to 7% due to its higher head and higher capacity. Therefore, an output frequency range and an output voltage range required for the secondary exciter are also reduced in proportion to the variable-speed operation range. Accordingly, even if the secondary exciter is connected to the secondary winding of the variable-speed generator-motor and used as a starter to supply driving electricity as heretofore, the generator motor cannot be started up to even the minimum speed of the variable-speed operation range due to a low output voltage.
In order to address such a problem, the following methods may be applied. That is, there may be a first method (static starter method) in which the static starter is additionally installed as described above; a second method in which a set of large-current branch circuit buses, disconnectors, and the like are provided, so that the main circuits are switched to inject driving electricity from the starter winding (primary winding) of the generator-motor (self-starting method [primary side]) at its starting; and a third method in which a start-up transformer, a set of large-current branch circuit buses, and disconnectors that are only used at the start are provided on the secondary winding circuit side, so that a secondary voltage at the start is started up, and necessary driving electricity is injected from the secondary winding circuit side (self-starting method [with a secondary start-up transformer]). In each case, large-sized equipments shall be additionally installed. This leads to increased equipment costs, complicated layout design of a power plant, and the increase in size and engineering costs of a building.
Under such circumstances, it is desired to provide a controller and a variable-speed generator-motor starting method whereby when a variable-speed generator-motor is started by a secondary exciter in a variable-speed pumped storage system, the variable-speed generator-motor can be started without increases in equipment size and costs even if the variable-speed generator-motor cannot be started up to the minimum speed of a variable-speed operation range.